Rare-earth-doped fiber lasers are finding a variety of uses especially in optical communication systems where they can be integrated effectively with fiber links, and active fiber devices such as erbium fiber amplifiers. Fiber lasers are typically laser pumped with inexpensive multi-mode lasers, such as GaAlAs, but have high power, single mode outputs. Fiber laser structures have relatively large active areas so that heating effects, known to be detrimental to the lifetime of other solid state laser structures, are largely absent. See e.g., L. Zenteno, "High-Power Double-Clad Fiber Lasers", Journal of Lightwave Technology, Vol. 11, No. 9, pp. 1435-1446, September 1993.
It is known that the power of fiber lasers scales well with cavity length. However, intrinsic losses in the host fiber material also scale with length, and these losses can vary (increase) over time giving unstable system performance. An attractive alternative for increasing power would appear to be to increase the active core area by increasing the core diameter thus increasing the pump absorbing area for a given fiber length. However, for single mode output, this option requires a low core .DELTA.. In a preferred structure, a threshold level of germanium dopant is desired in order to write Bragg gratings in the fiber core and thereby create a laser cavity. It is also known that dopants such as aluminum aid in solubilizing the active rare earth ions. Without an effective amount of Al for this function, the rare earth dopants crystallize, resulting in excessive scattering losses. However, both of these additives increase the core .DELTA.. To keep the overall level of index-modifying dopants low enough to satisfy the low core .DELTA. requirement mentioned above, the concentration of germanium and aluminum may be too low to meet the above mentioned goals. An approach that would accommodate these conflicting requirements and allow an increase in the active core diameter would represent a significant advance in this technology.